Testing and Inspection for Broken, Cracked or Chipped Spark Plug Insulator - Page 817
Spark Plug Overheating
Overheating is indicated by a white or gray center electrode insulator that also appears blistered. The increase in electrode gap will be considerably
in excess of 0.001 in. per 1000 miles of operation. This suggests that a plug with a cooler heat range rating should be used. Over advanced
ignition timing, detonation and cooling system malfunctions also can cause spark plug overheating. Also check for lean air-fuel mixture. Check the
torque value of the plugs to ensure good plug-engine seat contact.
